It is well known that helicopter flight is primarily controlled by cyclic and collective control inputs to the rotating helicopter blades, usually through a common mechanical input such as a swash plate. Input and transfer of these control signals to the rotating blades is most often accomplished by an exclusively mechanical arrangement of levers, mixers and cranks. Some known control systems modify the mechanical arrangements by the addition of a hydraulic boost mechanism to provide amplification of the mechanical input signals. Other control systems are mechanical with auxiliary hydraulic amplification, but lack a direct mechanical connection between the control stick that inputs the signals to control the helicopter blades.
These known control systems all must provide the same blade control signals to every blade because of the common control signal input mechanism. Individual blade control, wherein each blade may receive different control signals, is therefore physically impossible.
A major problem which still plagues present day helicopters is the very high vibration magnitudes. The major sources of these vibrations are rotor induced shears and moments. Consequently, known control systems incorporate mechanical devices such as vibration absorbers and isolators to provide compensation. Unfortunately, these mechanical devices do not effect the magnitude of the shears and moments.
The nature of the vibration inducing shears and moments is such as to produce an input at the hub at a frequency which is an integral multiple of the number of blades in the rotor system. Therefore, the predominant frequency of excitation is the "nth" harmonic of an n bladed rotor. For a four bladed rotor the predominant frequency of the vibration is the fourth harmonic or four per rev.
Other helicopter control systems have attempted to minimize vibration by reducing the magnitude of these shears and moments by the introduction of blade pitch changes at non-predominant harmonic frequencies. The airloads on each blade are therefore altered by the control system generating blade loads at other than the predominant harmonic frequency. For a four bladed helicopter rotor, these systems introduce three per rev and five per rev pitch changes on the blades in the rotating system. Introduction of these harmonic forces has been accomplished by superimposing a predominant harmonic (four per rev for four bladed helicopters) translation and rotation on the nonrotating control mechanism or swash plate at the helicopter blade hub. These higher harmonic input forces are introduced downstream of azimuth or the swash plate in the stationary part of the control system. Individual blade control, not physically possible with known helicopter control systems, provides tremendous versatility because other control harmonic forces (such as two per rev forces) which can also effect rotor performance, can be easily introduced on a per blade basis. With mechanical systems of the prior art, the mechanical control elements have inherent built-in drawbacks, such as backlash and bearings hystresis, due to the friction and structural compliance found in these elaborate mechanical systems, even when augmented by servohydraulics. With known mechanical systems the higher harmonic forces input to reduce vibration problems are all of small magnitude and operate at relatively high frequency. However, the benefits of the compensating harmonic forces are seriously diminished because the mechanical control system lacks sufficient resolution, and is most often saturated.
It would be advantageous for a helicopter control system to be fully hydraulic allowing for individual blade control and allowing for auxiliary blade tracking higher harmonic and trim force control inputs using exclusively electrical or optical input signals and hydraulic controls. The present invention is directed toward such a system.